Catalytic converters--metal foil material for use therein, and a method of making the material

ABSTRACT

A metal foil substrate material for catalytic converters and method of making the material in which layers of ferritic stainless steel and aluminum are solid state metallurgically bonded together forming a composite material. Such composite material is further rolled to the final foil gauge with no heat treatment and then subjected to a thermal in situ reaction to form a resulting uniform solid solution foil material with superior high temperature corrosion resistance.

This application is a division, of application Ser. No. 08/111,384, nowU.S. Pat. No. 5,366,139, filed Aug. 24, 1993.

BACKGROUND OF THE INVENTION

This invention relates to a composite material having corrosionresistance at high temperatures and method of manufacture and, moreparticularly, to the material and method for producing a metal foilmaterial for use in catalytic converters.

As well known, exhaust gases discharged from motor vehicles may containhalogen gases, halogen compounds and lead compounds, for example, Cl₂,Br₂, PbCl₂, C₂ H₂ Cl₂, C₂ H₂ Br₂ etc., besides unburnt noxious gasesincluding carbon monoxide, hydrocarbon and the like, and components orparts made of ferrous base alloy material for exhaust system of themotor vehicles or the like, for example, heat exchangers, air ducts,containers, etc., tend to be subjected to corrosion by the noxiouscompounds as described above. Moreover, halogen compounds (e.g. salt)employed for preventing freezing during cold seasons are liable to enterthese components of ferrous base alloy material, which are then corrodedby the atmosphere containing halogen gas produced when the halogencompounds are decomposed at high temperatures.

It has been known to use metal foil materials as substrates with anappropriate catalyst coating in place of ceramic material substrates.Such metal foil material has been made from steel sheets containingaluminum and also chromium in order to have the desired corrosionresistance at high temperature. These FeCrAl alloys, however, aredifficult to produce by conventional rolling and annealing processes. Toovercome the processing difficulties, it has been suggested, as in EPapplication 91115501.8, to produce the foil by a rapid solidificationprocessing method; but such processing is expensive and requires veryprecise controls. It has also been suggested to dip the stainless steelin a molten bath of aluminum or aluminum alloy to apply melt-plating onthe surface of the stainless steel (U.S. Pat. Nos. 3,907,611, 3,394,659and 4,079,157). This stainless steel with the aluminum is then subjectedto a heat treatment to form an alloy layer having Fe and Al as the maincomponents. Still further, surface layers of aluminum in bindermaterials, as described in U.S. Pat. No. 4,228,203, have also beensuggested. However, in all of these applications the control of theprocessing parameters is complex and expensive; and the final foil hasnot proven, in many cases, to have the desired corrosion/oxidationresistance at elevated temperatures.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides for a composite metal foilmaterial and method of manufacture typically for catalytic convertershaving corrosion resistance at high temperatures and resistance againstoxidation in an exhaust gas atmosphere. Further, the composite materialis easily and economically manufactured for high volume applications.

Briefly described, in accordance with the invention, a metal foilmaterial is made by producing a three layer material with the twooutside layers being essentially identical and chosen from the groupconsisting of stainless steel materials and aluminum materials; and thecentral layer being made from the material not chosen for the outsidelayers, as for example, a central layer of stainless steel sandwichedbetween two thinner outer layers of aluminum or aluminum alloys. Thethree layer materials, having a relatively thin starting thickness, arepressure rolled together to further reduce the thickness of the layermaterials, to metallurgically bond the layer materials to each other toform a composite multilayer metal material having a thickness such thatthe composite material can be pressure rolled to the final foilthickness without the necessity of any heat treatment operations. Suchcomposite is pressure rolled to this final foil thickness, and then isdiffusion heat treated at a temperature between 900° C. and 1200° C. fora period of time to cause diffusion of the various constituents in thelayers of the composite material throughout the foil. The compositeforms a material, with the complete presence of the constituents of thealuminum layer and the stainless steel layers dispersed throughout thetotal foil material thereby providing for the superiorcorrosion/oxidation resistance at high temperatures. The chosencomposition and thickness of these starting materials will provide for afoil material with a known furnished composition. The fact that noheating of the composite material is needed prior to reaching the finalfoil thickness, greatly increases the ability to manufacture thematerial with acceptable material yields in long continuous lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages and details of the novel material and methodof manufacture of this invention appear in the detailed description ofthe preferred embodiments of the invention, the detailed descriptionreferring to the drawings in which:

FIG. 1 shows a side elevation view diagrammatically illustrating thebonding method of this invention;

FIG. 2 shows the composite material of this invention after bonding;

FIG. 3 diagrammatically shows the material of this invention afterdiffusion heat treatment;

FIG. 4 shows the material used in a catalytic converter; and

FIG. 5 shows a photomicrograph of the material of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the novel and improved method and material of thisinvention, a first central layer 10 of ferrous material is sandwichedbetween two outer layers 12 and 14 of aluminum or aluminum alloymaterial. The three layers are passed between a pair of pressure rolls16 in a convention rolling mill 18. The layers are squeezed togetherwith sufficient force to be reduced in thickness, and metallurgicallybonded together along interfaces 20 and 22 between the metal layers toform a composite multilayer metal material 24 as shown in FIG. 2. Thematerial is then continuously rolled to the finished desired foilthickness and thermally reacted into a foil sheet 50 as will beexplained in detail below.

Typically, the first central layer 10 comprises a ferritic stainlesssteel with 16 to 24 wt. percent Cr, and the balance Fe with the Scontent less than 0.003 wt. percent. Examples of such ferritic stainlesssteels are 430, 434 and 446 stainless steels with controlled sulfurcontent. Preferably, top and bottom layers 12 and 14 are of the samethickness and material, and are comprised of essentially pure aluminumalthough aluminum alloys could also be used.

It is to be understood that the invention could equally well bepracticed with a central relatively thinner layer of aluminum oraluminum alloys, and top and bottom layer of the ferritic stainlesssteel material.

In a preferred embodiment having excellent high temperature oxygencorrosion resistance, it has been found desirable to have a finalchemistry in the final material 50 after thermal reaction (to beexplained in detail below) of between 18 to 20 wt. percent Cr, at least5 wt. percent Al and the balance Fe. Additionally, small amounts of rareearth metals such as Ce, Y, Er, etc., can be added to either of themetals forming the composite to also increase high temperature corrosionresistance. An example of such an embodiment is where a layer of 434stainless steel, having a thickness typically of between 0.045 and 0.070of an inch, is bonded to essentially pure aluminum top and bottom layershaving a thickness typically of between 0.004 and 0.007 of an inchthereby yielding a bonded composite of approximately 0.015 to 0.025 ofan inch as shown in FIG. 3. One typical example results in a compositeof about 84 percent stainless steel and a top and bottom layer ofaluminum of about 8 percent each. The initial starting thicknesses ofthe layers have been chosen to determine two important materialcharacteristics of the final composite. The first is to determine theultimate chemistry of the final composite after thermal reaction, andthe second is to provide a bonded composite which is capable of beingreasonably continuously rolled to the final desired gauge after bondingwithout the need of any thermal treatment which could cause theformation of intermetallics of the metal constituents. This second itemis particularly important in being able to produce the materialeconomically in large production quantities.

Accordingly, the composite 24 is cold rolled by Conventional means fromthe bonding gauge to the final foil thickness typically of about 0.002of an inch. This finish rolled foil is then thermally reacted or heattreated at a temperature between 900° C. and 1200° C., and preferablyabout or above 1100° C. for between 1 minute and 60 minutes or longer toprovide for diffusion of the various constituents in the compositethroughout the foil material. That is, after this heating operation, themicrostructure of the foil will not be a three layer structure; butinstead a uniform solid solution alloy. The heating can be done in avacuum, reducing atmosphere or air. The desirability of heating above1100° C. is the fact that such heating eliminates the formation ofundesirable Kirkendall voids, and ensures that all iron aluminideintermetallics are dissolved. The occurrence of Kirkendall voids is alsominimized by keeping the sulfur content of the composite, andspecifically the stainless steel component, to extremely low levels(preferably less than 0.003 wt. percent). These Kirkendall voids cancause premature failure of the foil in fatigue loading which easily canresult when such foil is used as a catalytic converter substrate.

The oxidation resistance of the foil material of the present inventionis further enhanced by the formation of a thin layer of an aluminumoxide whisker network on the metal surface typically done in anadditional heating operation at a temperature between 900° C. and 1100°C. in air although it may be done as part of the diffusion heating step.The oxide layer prevents the metal from further oxidation. Duringthermal cycling when used as a catalytic converter substrate foilmaterial, the oxide layer tends to crack and separate from the basematerial directly below the surface which results in this materialfurther oxidizing. To reduce the separation of the surface oxide layerfrom the base material below and to increase adhesion, a small amount ofrare earth elements (between 0.01 and 0.10 wt. percent) is preferablyadded to the foil material. In the present invention, this rare earthmetal addition can be made by the addition of such metal(s) in eitherthe starting aluminum or ferrous materials. These rare earth metalelement additions, combined with extremely low sulfur content, furtherminimize any Kirkendall voids.

In order to give greater appreciation of the advantages of theinvention, the following examples are given:

EXAMPLE I

A continuous strip of completely annealed 434 stainless steel having athickness of 0.060 of an inch was cleaned and brushed. This strip wassandwiched between two continuous strips of cleaned aluminum foil of0.006 of an inch each and roll bonded in a single operation to yield asolid state metallurgically bonded three layer composite of 0.020 of aninch as described in U.S. Pat. No. 2,753,623 which is incorporatedherein by reference. This composite material continuous strip was coldrolled on a conventional rolling mill in multiple passes until the finaldesired gauge of 0.002 inches was achieved. This foil material was thenheated to 1100° C. in vacuum for 60 minutes to diffuse all the aluminuminto the stainless steel base, thereby forming a complete uniform solidsolution foil material. This foil material showed approximate chemicalcomposition of 75 wt. percent Fe, 20 wt. percent Cr, and 5 wt. percentAl. These metal materials listed above were of uniform concentrationthroughout the foil.

EXAMPLE II

This example was carried out identical to Example I above except thestarting center strip used was pure aluminum having a thickness of 0.012of an inch and the top and bottom strip layers were of 434 stainlesssteel each having a thickness of 0.030 of an inch. The finished foilmaterial after heat treatment had the same uniform solid solutionchemical composition as set forth in Example I above.

EXAMPLE III

This example was carried out identical to Example I above except the 434stainless steel was replaced by a stainless steel of the followingcomposition: 20 wt. percent Cr, 0.015 wt. percent Ce, 0.004 wt. percentLa, 0.009 wt. percent Ni, 0.002 wt. percent S and the balance Fe withthe unavoidable impurities of Mn, and Si. This finish foil materialafter heat treatment had a uniform solid solution chemical compositionapproximately of 73.3 wt. percent Fe, 18.2 wt. percent Cr and 6.7 wt.percent Al for the major constituents with S and rare earth metals inthe desired range.

EXAMPLE IV

This example was carried out identical to Example III above except thefurnished foil material was further heat treated in air at 960° C. for20 hours to produce the alumina whisker network, Such alumina whiskernetwork is desirable for applying ceramic wash coat on the foilsubstrate as commonly practiced for catalytic converter manufacture.

Accordingly, the foil for use as a substrate in catalytic convertersprovided by the process of this invention is typically provided with analumina whisker network on the surface of the foil. This alumina whiskernetwork (not shown) has a ceramic wash coat, as is known in the art,applied on the foil substrate (not shown), and such catalyst sheet 51(with whisker network and wash coat) is positioned on a frame 52 to forma catalytic converter unit 54 as shown in FIG. 4.

The novel process and article produced by method of the presentinvention provides for a foil material for use in catalytic converterswith good corrosion resistance at elevated temperatures. The material iseasily and economically manufactured having a selectively predetermineddesired chemical composition. The chemical composition is uniformthroughout the foil sheet. While the invention has been described incombination with the specific embodiments thereof, it is evident thatmany alternatives, modification and variations will be apparent to thoseskilled in the art in light of a foregoing description.

We claim:
 1. A foil substrate material having various metal constituentsfor catalytic converters made by the process of providing a layer of afirst material chosen from the group consisting of chromium containingferrous metals and aluminum and aluminum alloys, sandwiching said layerof first material between first and second layers of a second materialchosen from the group consisting of chromium containing ferrous metalsand aluminum and aluminum alloys not chosen for the first material,metallurgically bonding said layers together by reducing the thicknessof said layers thereby forming a multilayer composite material of saidfirst and second materials, reducing the thickness of the compositematerial to a final thickness and heating said material composite insitu at a temperature between 900° C. and 1200° C. for a sufficientperiod of time to cause diffusion of the various metal constituents ofsaid layers throughout the composite material thereby providing auniform solid solution material for the foil substrate.
 2. The foilsubstrate material according to claim 1 wherein said uniform solidsolution material is further heat treated in air at a temperaturebetween 900° C. and 1100° C. for sufficient time to form a thin surfacelayer of an aluminum oxide whisker network on the surface of the uniformsolid solution material.
 3. The foil substrate material according toclaim 2 in which a ceramic wash coat is applied to the surface of saidaluminum oxide whisker network.
 4. A catalytic converter comprising aframe with a plurality of layers of foil substrate material of claim 3.